You could try shell dwellers. They have a minimum of 5 gallons for a pair. You have to have a sand substrate, but if you put a stocking or sponge over your intake tube on your filter, that will prevent sand from being drawn into your impeller. They like a temp of 76-79 degrees, and a ph of 7.8-8.6.
Just do a search on shell dwellers. Lots of available info out there.
Jan